1. Field of the Invention
The present invention relates to metal-oxide-semiconductor (MOS) devices and, more particularly, to a method of growing MOS gate oxides.
2. Discussion of Related Art
Perhaps the most important application of thermal oxidation to ULSI processing is forming of the thin gate insulating layer of MOS devices. For example, gate oxide layers thinner than 150 .ANG. are required for MOS transistors with gate lengths below 1 .mu.m, while gate oxides thinner than 80 .ANG. are required for the tunnel oxides of electrically alterable read-only memories.
Since the silicon dioxide (SiO.sub.2) layers are, in these cases, components of active devices, the thin oxide layers must be uniform, of high quality, and must be formed by means of a sufficiently slow process so that the oxide thickness can be reliably controlled. Control of gate oxide thickness is particularly important, since the threshold voltage of an MOS device depends on the thickness of its gate oxide. The difficulty of reliably controlling gate oxide thickness is moreover compounded when gate oxides of two different thicknesses must be formed during the manufacture of a single device.
A conventional method for forming thin MOS. gate oxides having two different thicknesses is described immediately below with reference to the idealized cross-sectional views of FIGS. 1A-1D.
Thick field oxides 11 on a silicon substrate 10 define first and second active (i.e., device) regions of the substrate. The first active regions are defined as those active regions over which the thicker of the two subsequently formed gate oxides is formed, while the second active regions are defined to be those active regions over which the thinner of the two gate oxides is formed.
After first oxides 12a and 12b have been thermally grown at the surface of the substrate over the first and second active regions, respectively, a photoresist coating is spun onto the wafer, photoresist pattern PR1 which masks the first active regions is formed by means of conventional exposure and development process steps, as shown in FIG. 1B.
Masked by the photoresist pattern PR1, the substrate 10 is selectively etched to remove the first oxides 12b over the second active regions, thereby exposing the surface of the substrate 10 over the second active regions, as shown in FIG. 1C. The photoresist pattern PR1 is then stripped so the first oxides 12a over the first active regions is thereby exposed. Second oxides 13a and 13b are then thermally grown at the surface of the substrate over the first and second active regions, respectively, as shown in FIG. 1D. Each of the second oxides 13b over the second active regions will subsequently serve as a thinner gate oxide 14b of an MOS device. At the same time, the second oxides 13a over the first active regions are thermally grown at the surface of the substrate under the first oxides 12a over the first active regions. Each of the first oxides 12a over the first active regions and the second oxide 13a grown under it will together subsequently serve as a thicker gate oxide 14a of an MOS device. (The second oxides 13a over the first active regions grow under the first oxides 12a over the first active regions, rather over the first oxides 12a over the first active regions, because the oxidant, rather than silicon, diffuses through the first oxides 12a during the course of thermal oxidation of the silicon substrate.)
Conventional methods of forming gate oxides of two different thicknesses on a silicon substrate, as typified by the sequence of process steps described above, require two separate thermal oxidations. Moreover, since the first photoresist pattern PR1 is formed on the first oxides 12a over the first active regions and the second oxides 13a over the first active regions are grown under the first oxides 12a, the thicker gate oxides 14a, each of which comprises a second oxide 13a and the first oxide 12a over it, are inevitably contaminated by organic and metallic remnants of photoresist.